Agas with a temperature of 21.0°c and a volume of 10.0 l is compressed to 5.00 l. what will be the new temperature?

In order to compute the final volume, we may apply Charle's law, which states that for a given amount of gas at constant pressure, the volume and temperature of that gas are directly proportional. Mathematically, this is written as:
Volume / Temperature = constant
Note that the temperature is measured on the absolute scale whenever applying gas laws. Thus:
10 / (21 + 273) = 5 / (T)
T = 147 K

The new temperature will be 147 K or -126 °C